Chapter 10 Energy, Work and Simple Machines

Energy The ability to produce change in itself or its’ environment.

Energy of Motion: Kinetic Energy

Newton's 2 nd Law F = ma Equation for an object in motion. v 1 2 = v ad F m = a

Kinetic Energy v 1 2 = v Fd m v v 0 2 = 2Fd m ½m(v v 0 2 ) = Fd

Kinetic Energy Fd = Work ∆K = Work

Kinetic Energy ½mv ½m v 0 2 = Fd K = ½mv 2 ΔK = Fd

Work Applying a Force through a distance is WORK!! Fd = Work ΔK = Work

Work Work is measured in Joules(J). J = Nm J = kg m 2 s 2

Example 1 A 105g hockey puck is sliding across the ice. A player exerts a 4.5N force over a distance of 0.15m. How much work does the player do on the puck? What is the change in the pucks energy?

Example 1

Given: m=105g=0.105kg F=4.5N d=.15m Eq:W=F d W=(4.5N)x(0.15m) W=0.68J

Example 2 A gardener pushes a lawn mower across a 20m lawn. He applies a 250N force to the handle. If the handle makes a 30º with the ground, how much work is done to make one pass across the lawn?

Example 2 30º F d

Example 2 x comp:F x = Fcos30º Given:d=20m F=250N at 30º F x = 250cos30º F x = 217N Eq:W = F x d W = 217N X 20m W = 4340J

Homework 10-1 Practice Problems: 1-5 Page: 227 Practice Problems: 6-8 Page: 229 Due: 1/27/03

Power – watts - W Power is the rate at which work is done. Power = work time

Power P = W t P = Fd t

Example 2 A farmer lifts a 1.0kg bail of hay 2m. On a good day it takes the farmer 0.5s. How much power does the farmer produce.

Example 2 Given:d = 2m m =1kg t = 0.5s Eq:P = W t P = mgd t P = Fd t

P = (1kg)(9.8m/s2)(2m) 0.5s P = 39.2 W P = mgd t

Homework Problems: 25,26,29 & 30 Page: 242 Due: 2/1/07

Machines A machine eases the load by changing either the magnitude or the direction of a force as it transmits energy to the task.

Mechanical Advantage FrFr FeFe

Effort Force ( Fe ) : The force you apply to the machine. Resistance Force ( Fr ) : The force exerted by the machine.

Mechanical Advantage Mechanical Advantage (MA) The number of times the machine multiplies the effort force. MA = resistance force effort force MA = Fr Fe

Ideal Mechanical Advantage drdr de Fr Fe

Ideal Mechanical Advantage Work In = Work Out IMA = d e d r

The Lever Fr Fe lr le

MA lever = effort arm length resistance arm length MA lever = l e l r

Pulleys Fixed Pulley Movable Pulley

Mechanical Advantage of a Pulley: The number of ropes supporting the resistance weight. MA = 1MA = 2MA = 3

Block and Tackle The arrangement of several pulleys.

Wheel and Axle rwrw rara MA = radius of wheel radius of axle MA = r w r a

Inclined Plane A slanted surface used to raise objects l h MA = l h

Efficiency Efficiency = Work Out x 100% Work In Efficiency = F r x d r x 100% F e x d e

Compound Machine MA = MA 1 x MA 2 1 2

Homework 10-3 Practice Problems: Page: 238 Section Review: 1-4 Page: 238 Due: 1/30/03

Homework 10-4 Problems: 31,33,37, 40, and 41 Pages: Due: 1/31/03 Test: 2/4/03

Homework 10-5 Problems: 48, 49, 50, 52, and 53 Pages: Due: 2/3/03 Test: 2/4/03